1. Field of the Invention
The present invention relates to an image reading apparatus for image-processing signals obtained by reading an original image and outputting image signals corresponding to the original image.
2. Description of the Background Art
In general, an image reading apparatus (image reader) for subdividing an original image into picture elements and reading the same is employed as an image input device for an apparatus such as a digital copying machine or facsimile, an OCR (optical character reader), an optical filing system or a display unit for forming, recording or displaying an image by combination of dots (picture elements) of constant sizes.
In such a conventional image reading apparatus, a one-dimensional image sensor extending in a main scanning direction is employed to scan an original image in a subscanning direction for reading the same, convert the as-obtained analog signals of respective picture elements to image data of multilevel digital signals, image-process the converted image data and output binary image data corresponding to the respective picture elements.
Image processing performed by such an image reading apparatus includes processing for improving image quality such as edge (contour) enhancement for making a clear image or smoothing for making a natural image, processing for image editing such as trimming, masking, negative-positive conversion, and binarization processing for converting multilevel data responsive to densities or colors of respective picture elements of an original to binary data.
In the binarization processing, the so-called simple binarization processing for comparing the multiple level of a single picture element with a constant threshold level is suitable for a line image such as a character in general, while pseudo gradation processing by a dither method or an error diffusion method is suitable for a half tone image such as a photograph, in consideration of the quality of the reproduced image.
In order to read an original containing both line and half tone images, therefore, the simple binarization processing and the pseudo gradation processing are switched in response to the attribute (this term may hereinafter be referred to as "binarization attribute") of the line or half tone image.
In the conventional image reading apparatus, image data obtained by reading an original are image-processed by a single image processor. For example, a simple binarization processing unit and a pseudo gradation processing unit are respectively provided for binarization processing, to perform simple binarization processing or pseudo gradation processing by one of the processing units on the image data inputted in parallel with reading of the original image.
However, the binarization processing requires a considerable time for fully comparing image data of respective picture elements with threshold levels. Particularly in the pseudo gradation processing, it is necessary to read threshold levels which are varied with positions (addresses) of the picture elements from a ROM or the like. Thus, the processing speed cannot be increased due to the time required for such an operation.
When an original contains a large amount of half tone images, therefore, the overall processing time is increased and the reading speed is reduced. When a large original such as an A2 original is read, for example, the reading speed is reduced due to increase in number of picture elements.